Carbon sequestration

Over the past few years we have heard a lot about “clean
coal” technologies. Clean coal refers, supposedly, to methods
of burning coal without producing greenhouse gases.

Coal is a material that is almost pure carbon. There are some
impurities, of which the one of greatest concern is sulphur. If we
burn sulphur we get sulphur dioxide (SO2), a gas. The
sulphur dioxide will combine with atmospheric moisture to produce
sulphurous acid (H2SO3) or sulphuric acid
(H2SO4). These are both undesirable, because
they produce “acid rain” that destroys vegetation.

[Aside: the word “sulphur” is nowadays more commonly
seen with its American spelling “sulfur”. I'm
old-fashioned, so I'll stick with the spelling I'm used to.]

A few years ago, the term “clean coal technologies”
would have referred to the use of coal with low sulphur content; to
methods of removing sulphur from coal; to methods of removing sulphur
dioxide from power station exhaust gases; and related issues. By now
the sulphur problem has largely been resolved, but a new problem has
arisen. If we burn pure carbon, with no impurities, the result is
energy plus carbon dioxide. The relevant chemical equation is

C + O2→
CO2

where C means carbon, O2 means oxygen, and CO2
means carbon dioxide. The problem we now face is that carbon dioxide
is a greenhouse gas: a gas whose presence causes global warming.
Global warming threatens our quality of life, and perhaps even the
survival of humanity on earth, so we worry about it.

Greenhouse gases

The temperature of the earth, at ground level, depends on multiple
factors, and I can give only a broad overview here. Broadly, the
earth is heated by radiant energy from the sun, and it is cooled by
re-radiation of energy back to outer space. Obviously there are local
effects – the heating is greatest at the tropics, least at the
poles, and the heat is redistributed by winds and ocean currents –
but for our present purposes it is sufficient to think in terms of a
global total and a global mean temperature.

The heat that matters to us corresponds to solar radiation in the
infrared part of the electromagnetic spectrum. We can't see infrared
light, because our eyes don't work properly at infrared frequencies;
but we can feel it, because a lot of materials, including our skin,
are good at absorbing radiant energy at infrared frequencies.

The term “greenhouse gas” comes from the observed fact
that the temperature inside a greenhouse is (intentionally) higher
than it would be if the greenhouse were not enclosed. This rise in
temperature is mainly due to an elevated level of carbon dioxide
inside the greenhouse. Similarly, if the amount of carbon dioxide in
the earth's atmosphere is increased, the mean global temperature is
going to rise. Some local temperatures might fall, but the average
will still increase.

Some materials absorb and retain heat, while others are either
transparent to it or reflect it. (And most materials lie somewhere
between these extremes.) The retention depends on the relationship
between the energy states of molecules and the wavelength of the
incident radiation. In the case of atmospheric gases, we can observe
that some gases are transparent to solar radiation, while others
absorb it (and therefore increase their temperature). The gases that
contribute most significantly to global temperature increases are (in
order of importance) water vapour, carbon dioxide, methane, and
ozone. The other important gases in the atmosphere – oxygen,
for example, or nitrogen – are less of an issue because of
their molecular size: they are not important absorbers in the
infrared spectrum.

Water vapour is pretty much uncontrollable. Our oceans provide an
almost inexhaustible supply of water, and the amount of that water
that turns into water vapour depends on the atmospheric temperature.
We could conceivably hope to control the temperature, but we can't
hope to control the amount of available water.

Ozone is not an enormous issue because there just isn't a great
deal of it in our atmosphere. In any case, we don't want to reduce
the amount of atmospheric ozone because the ozone plays a valuable
role in shielding us from ultraviolet radiation from the sun.

That leaves carbon dioxide and methane as the gases that we need
to worry about. Methane is the product of many biological processes –
the belching of cattle being a particularly important source –
and molecule-for-molecule it's actually a more important greenhouse
gas than carbon dioxide. Despite this, carbon dioxide is the bigger
factor because there's more of it in the atmosphere.

More importantly, carbon dioxide levels are more sensitive to
human activity. If we burn organic materials, such as wood and oil
and coal, we will increase the production of carbon dioxide. The
reason why this matters is that we, as a species, use a lot
of electricity, and that means a lot of carbon dioxide produced by
power stations that burn fossil fuels.

Is carbon dioxide
really all that bad?

We humans have evolved in an atmosphere
that contains about 0.038% (by volume) of carbon dioxide. We actually
rely on it. If there were no carbon dioxide in the air, we might even
die, because our breathing reflex depends in part on carbon dioxide
levels. It is not at all a “bad” gas.

Carbon dioxide is part of the natural
respiration cycle of most earthly animal and plant species. Most
animals, including humans, breathe in oxygen and expire carbon
dioxide. Plants do something similar, but plants also use a
photosynthesis operation that consumes carbon dioxide –
converting the carbon to organic matter – and releases oxygen.

The overall balance of gases is tied in
with the mix of species. An increased carbon dioxide level will
probably lead to increased plant growth, partly because of the
increased level of available carbon and partly because of an increase
of temperature. (Because more carbon dioxide means more absorption of
solar radiation.) At the same time, there is likely to be a change in
the mix of animal species, because of the combined effects of the
change in breathability of the atmosphere and (probably more
importantly) the mean temperature of the atmosphere. As a first
approximation, we would expect that an increase in CO2
levels would mean a growth in reptile numbers and a drop in mammalian
populations. If the change is big enough, some mammals –
perhaps including humans – would go extinct.

If the change is not too abrupt, the
species mix would change smoothly to adapt to the change in
conditions. For those who believe in the Gaia hypothesis, Gaia would
survive and adapt. It is important to understand, however, that
humans do not occupy a privileged position in the mixture of life on
earth. It is certain that Earth can survive even a major change in
the concentration of greenhouse gases. It is less certain that
humanity would be one of the surviving species.

Sequestration

If we decide that burning coal is a
dangerous thing to do, then we need also to decide what to do about
our coal-burning power stations.

The “clean coal”
philosophy would, if taken literally, lead for a search for a way of
burning coal that did not
produce carbon dioxide as a product. This in turn suggests that we
need to find a way of removing all of the carbon from coal. Since
good-quality coal is almost pure carbon, this is unlikely to work.

We can expect, therefore, that
the clean coal advocates will accept that the production of CO2
is inevitable, and that they will instead look for ways of stopping
the CO2
from escaping into the atmosphere. Most of the obvious ways –
for example, the conversion of CO2
to liquid carbonic acid – create further more tricky problems,
so the clean coal researchers have a difficult road ahead of them.
The difficulty is, however, mitigated by the fact that mine owners
are very strongly motivated to pour money into clean coal research,
and do not particularly care whether that research is likely to yield
any results, provided only that it keeps going.

The most promising approach, as of the
date of this article, is carbon sequestration. Sequestration refers
to the approach of putting the product of combustion – whether
it be carbon dioxide, or something else – into a place where it
is not released into the atmosphere. This might, for example, be some
sort of underground reservoir. When the stored gas is finally
released – by earthquake, or melting of sea ice, or some
similar process – it will have a much higher concentration, to
the point where it could cause mass species extinctions. The
expectation, however, is that this will probably not happen until
after the expiration of the term of office of all current
governments.

A simpler solution

There is already a natural way of removing carbon dioxide from the
air. Plants consume carbon dioxide, and turn it into solid carbon
compounds. Some of the plants are consumed by animals, including
humans, and again much of the carbon remains locked as solids inside
the body. (Although we do have to worry about gaseous by-products of
digestion. Methane, in particular, is a worse greenhouse gas than
carbon dioxide, and it enters the atmosphere when animals fart and
belch.) The remaining plants die, and if we're lucky they retain
their carbon as solids rather than gases. We just have to be careful
not to burn the wood, because that would again release undesirable
things into the atmosphere.

In the really long term, some of that decayed vegetable matter is
trapped underground, and gets transformed into coal and oil. This is
nature's purest form of carbon sequestration. It has finally turned
atmospheric carbon compounds into liquids or solids that are trapped
underground and will not evaporate.

Of course, we have to make sure that we don't remove this oil and
coal from the ground. That would undo the work of millions of years.